Computer devices, such as laptops and the like, are often equipped with touch sensitive sensors or navigation devices. These are typically CMOS devices which include an array of pixels acting as an image sensor. The image sensor detects movement of an object in the vicinity thereof and converts that detected movement into control signals for controlling the computer device.
Some CMOS image sensors include a rolling blade shutter in which the pixels are processed line by line, one being integrated and another being read out for each movement of the shutter. The shutter moves over the array so that the pixels are exposed for the same amount of time, but not all at the same time. Rolling blade shutter does not work well when taking images of fast moving objects as is illustrated by the imaging problems associated with fans, helicopter blades or propellers. Operating a rolling blade shutter for most types of consumer use is not generally a problem and in fact most users do not notice these types of artefact.
However, in the area of machine vision where the images are processed and analyzed by a computer algorithm, distortion of an image due to motion can be a major drawback. In these cases the algorithms will typically fail or produce inaccurate or unreliable data. As a result, in machine vision types of application, a pixel array with global shutter is generally used. Here, pixels are simultaneously released from reset and start to integrate simultaneously. After a specific period, the pixels are then read out simultaneously into a temporary storage, which is often located inside the pixel. This temporary storage is then scanned out row by row where the signal is amplified or converted into a digital value.
Pixels for rolling blade shutter operation typically have a low number of transistors and so are less dense and suitable for sensors with large number of pixels where the size of the die and the size of the image plane should be kept small.
Currently, global shutter pixels have a single storage element in each pixel. Hence after an image has been acquired, it is to be read out before the next signal can be acquired. This typically involves an amplification process and/or a conversion into a digital value. The readout time is typically significantly longer than the exposure time. For example, a 640×480 pixel array can be read out at 200 Hz (5 ms) while the exposure time could be as short as 10 μs to prevent or reduce motion blur. Hence, with a single storage element in each pixel, two exposures could be as much as 5 ms apart.
The type of delay caused by readout of the array can cause issues in certain circumstances. For example, if background or ambient illumination is to be removed from an image, the amount of ambient illumination (e.g. due to flicker from mains-powered lamps) could change considerably in 5 ms. Similarly, if motion detection is desired, then the image may have moved considerably in 5 ms, making it harder to determine the direction of movement, and potentially making the system sensitive to temporal aliasing. Temporal aliasing occurs if a structure with a repetitive spatial frequency such as a barcode or propeller blade has moved almost one complete cycle during the delay between frames, at which point the second image will appear to have moved backwards.
To avoid these types of issues, it is desirable to be able to take two images in very quick succession especially for ambient light detection and movement detection.
U.S. Pat. No. 7,375,752 discloses a global shutter implementation suitable for small image array sizes (e.g. 20×20 pixels) in an optical mouse suitable for storage of multiple images per pixel. However, as there are as many interconnection wires between the array and the readout as there are pixels in the array this technique would not work with larger arrays, as the wires would block light from falling onto the array.
Various circuit architectures have been proposed to attempt to develop a global shutter application which can be used to store one image or a number of subsequent images.
A three transistor (3T) pixel architecture has been proposed which provides gain within the pixel and so has low noise. It can also be fabricated in most CMOS process technology without any modification to the process. However the architecture suffers from issues associated with dark current and reset (kTC) noise.
A four transistor (4T) pixel architecture is proposed having a buried photodiode so that the architecture is immune to dark current generated at the surface and hence has a lower dark current than the photodiode on a 3T pixel. The pixel is engineered so that it can be fully depleted and as such there are no electrons stored on it during reset and so is immune to kTC noise. This architecture is adopted for systems with small pixel size and where the system is to operate in low light levels as it has lower noise than a 3T pixel architecture.
The 4T pixel architecture does not work well under high light levels as once the charge on the photodiode has decayed to ground, further photo-generated charge will not be stored in the photodiode but can flow to adjacent pixels, degrading the image in a process called “blooming”.
A five pixel architecture (5T) has been proposed which addresses the issues of blooming under high light levels by using another transistor to provide a path for photo-generated electrons when the pixel is not integrating. However, the 5T architecture has a single storage node and so cannot acquire two frames quickly. In addition, the voltage stored on the sense node is affected by dark current or parasitic light sensitivity. Typically the capacitance of the sense node is kept low to increase the conversion gain and hence a small level of dark current or photo-generated charge will have a large effect.
A seven pixel (7T) architecture has been proposed which overcomes the issue with parasitic light sensitivity and dark current by having a source follower in the pixel and storing the signal on a larger capacitor. However, this pixel is not suitable for rapid acquisition of images as the image still needs to be read out from the array before the next one is acquired.
X. Wang, J. Bogaerts, et. al. have proposed an eight pixel (8T) architecture in SPIE Electronic Imaging 2010 vol. 7536: “A 2.2 μm CIS for machine vision applications”. This pixel has 3 storage nodes in the form of a floating diffusion and two capacitors. The second capacitor allows for greater suppression of parasitic light sensitivity but does not allow for speedier image acquisition.
A photo-gate pixel is disclosed in U.S. Pat. No. 5,471,515 and is similar to the widely used 4T pixel, except that it uses a photo-gate in which a potential voltage is applied to form a “potential well” so that photo-generated charge can be accumulated. This has a lower dark current than the 3T pixels but is no longer used as the reset (aka “kTC”) noise cannot be cancelled and the photo-gate reduces the sensitivity of the pixel to light, especially in the blue region of the spectrum. Although the photo-gate pixel is capable of global shutter operation, it has only 1 storage capacitor and thus also suffers from blooming.
The photo-gate pixel can be adapted to have two readout channels and could be used to acquire two images with very short (<10 μs) interval as described in U.S. Pat. No. 6,838,653. However it is difficult to manufacture this architecture as the photo-gate is typically overlapped by the two transfer gates to ensure photo-generated charge is transferred efficiently. This overlapping of polysilicon is not common in either standard CMOS or imaging process technologies and is difficult to produce. Further, pixels are particularly sensitive to parasitic light sensitivity and blooming as there is no way to reset the photo-gate without also resetting the floating diffusion capacitor where the signal is stored. If the photo-generated charge from one image is not completely transferred on read of the image then the charge will be transferred on the next image causing temporal noise in the next image.
As a result, there is no satisfactory design providing a sensor having a global shutter arrangement and having fast capture of two images for the purposes of ambient light sensing and/or movement detection.